The invention relates to mixing of orthopedic bone cement, particularly to components and methods for mixing bone cement. The disclosed invention equally well relates to mixing of dental and other similar cements.
Cement used as a casting and grouting material to implant prosthetic devices into live bone is made from a very fine cement powder, typically polymethyl methacrylate, mixed with a monomer liquid, typically methyl methacrylate, to form a flowable bone cement mixture. Physical mixing of the dry cement powder and liquid is required in order to make a flowable cement. It is not sufficient to merely bring the liquid into contact with the cement powder because the liquid will not flow into the powder uniformly. During mixing the monomer liquid should be distributed equally throughout the mixture so that the mixture is uniform and possesses a uniform viscosity or thickness, consistent with the manufacturer""s specifications.
Bone and dental cements are mixed using pre-packaged amounts or doses of dry bone cement powder and monomer liquid prepared by the manufacturer of the cement. The amounts of powder and liquid are measured to provide a cement mixture having desired properties when the powder and liquid are uniformly mixed together. Failure to mix the liquid and powder together uniformly means that part of the mixture contains an excess of monomer liquid and is runny and part of the mixture contains a deficiency of monomer liquid and is thicker than desired or, in some cases, retains unwetted dry powder.
It is quite important that the powder and monomer liquid be evenly mixed throughout the bone cement mixture. Bone cement with an excess of monomer liquid sets up slowly requiring increased operating room time and the risk of prosthesis displacement during protracted hardening or set-up of the cement. Such cement also possesses reduced strength. Further, during the increased set-up period there is a risk that blood or other bodily fluid will displace runny cement from adjacent the formed bone surface at an implant site and thereby weaken fixation between the hardened cement and bone.
Bone cement mixed with a deficiency of monomer liquid has a high viscosity and may be difficult to flow properly to the application site. This cement does not flow readily into the irregularities of a prepared bone surface to form a reliable joint. Further, bone cement with a deficiency of monomer liquid sets up relatively rapidly, reducing the already short amount of time available to the surgeon to perform an implantation procedure.
Bone cement is conventionally mixed in a bone cement mixer. The mixer has a funnel, a mixing chamber, a cover to close the chamber, and a mixing element or stirrer attached to the cover to mix the powder bone cement and monomer liquid together. Cement is typically mixed by placing the funnel on top of the mixing chamber, which may be the interior of a syringe cartridge, and pouring the required amount of bone cement powder through the funnel into the chamber. Monomer liquid is poured into the funnel and flows into the chamber on top of the powder. The funnel is removed and the cover is placed on the top of the mixing chamber. The cover closes the mixing chamber and inserts the stirrer into the cement powder. The stirrer is manually rotated in the chamber for a predetermined amount of time to mix the powder and liquid together and form a uniform, flowable bone cement. Insertion of the stirrer into the body of bone cement powder before monomer liquid is poured onto the powder compacts the powder and makes uniform mixing difficult.
It is important that the monomer liquid poured into the chamber be distributed in the powder to the extent that stirring will create a uniform, homogeneous bone cement mixture. The monomer liquid should preferably penetrate to the bottom of the powder body prior to stirring to ensure stirring will form a uniform mixture.
When the monomer liquid is poured on the powder, however, the liquid can form a puddle which wets the top of the powder. The wetted top layer of powder seals the powder and prevents air within the underlying powder from escaping. The liquid cannot penetrate deeply into the powder, because the liquid cannot displace the air within the powder. Hence powder at the bottom of the powder body remains dry. Stirring then will not uniformly mix the liquid and powder. Some powder may remain totally dry while other powder may be mixed with an excess of monomer liquid.
The just described problem of achieving desirable monomer liquid distribution prior to stirring is well recognized. This is particularly a problem where the powder placed in the chamber forms a tall column of powder, such as when the height of the powder column is greater than its diameter. This is typically the case when the cement is mixed in a tall, narrow bone cement cartridge of the type used to extrude mixed cement to an application site. The taller the column, the more difficult it is for the monomer liquid to penetrate to the bottom of the powder.
A conventional method for achieving the desired penetration of monomer liquid into the powder column when using a syringe cylinder as a mixing chamber requires pouring cement powder and monomer liquid alternately, in small amounts, into the chamber until the total amounts to be mixed have been poured into the chamber. The powder and liquid are then mixed. While this method can achieve improved uniformity of monomer liquid distribution, it has serious disadvantages. First, it consumes valuable extra time, and the duration of the time consumed results in a mixture which is not homogeneous in terms of the starting time of its mixing and, hence, its setting time. Second, the layering of monomer liquid which results from this method can isolate regions of loosely packed dry powder containing air which can form air inclusions in the mixed cement.
Other methods for achieving the desired penetration of monomer liquid into the powder column when using a syringe cylinder as a mixing chamber are known. Tilting or inclining the cylinder immediately before pouring the monomer liquid into the chamber promotes monomer liquid flow to the bottom of the powder. However, the powder cannot be compacted by prior handling or storage. Alternatively, a stream of monomer liquid can be flowed into the powder at high speed. This requires a mixer of relatively large size and complexity.
Thus, there is a need for an improved bone cement mixer and method which distributes monomer liquid poured on a column of bone powder in a tall, narrow bone cement mixing chamber, such as a bone cement cartridge, so that stirring will create a uniform, homogeneous bone cement mixture.
The invention relates to bone cement mixers in which bone cement liquid is flowed into dry bone cement powder, components and methods for mixing bone cement powder and monomer liquid wherein the monomer liquid poured on the bone cement powder prior to mixing is distributed through the column of bone cement to the extent that stirring will create a uniform, homogeneous bone cement mixture.
The cement is preferably mixed in a high, narrow bone cement cartridge. The bone cement powder is poured into the cartridge and forms a body of powder surrounded by the wall of the cartridge. A stream of monomer liquid is then flowed by gravity through a nozzle that aims down onto the top of the body of bone cement powder close to the cartridge wall. Monomer liquid from the gravity stream impacts the bone cement powder and is absorbed into the powder surrounding the stream. The liquid absorbed into the powder flows outwardly from the stream and attempts to form a cylinder of monomer-rich powder surrounding the stream. The remainder of the powder remains dry prior to mixing.
The nozzle is located so that the stream of monomer liquid is spaced a short distance from the wall of the cartridge to impact near the outer periphery of the body of powder. The stream is spaced sufficiently close to the outer periphery of the powder body such that some of the absorbed liquid flowing outwardly from the stream reaches the outer periphery of the powder body and is blocked by the wall. The wall prevents further outward flow of the liquid and prevents a cylinder of monomer-rich powder from surrounding the stream. Instead, a partial cylinder of monomer-rich powder forms against the wall and surrounds the stream. The partial cylinder of monomer-rich powder extends to the bottom of the powder.
Flowing the stream of monomer liquid near the outer periphery of the powder column to form a partial cylinder of monomer-rich powder enables a gravity-flowed stream to penetrate to the bottom of the powder without forming a puddle of bone cement liquid on top of the bone cement powder. The bone cement liquid is distributed essentially uniformly along the height of the bone cement powder column while maintaining the bone cement powder in the column remote from the stream dry.
After flowing the monomer liquid in the column, the bone cement powder and monomer liquid are mixed conventionally to form bone cement having an essentially uniform viscosity. The bone cement cartridge is then placed in a conventional bone cement gun for extrusion to an application site.
In a preferred embodiment, the nozzle is located below a liquid well that has an open upper end for receiving monomer liquid. An outlet passage flows the liquid in the well by gravity to the nozzle. The nozzle extends downwardly and includes a vertical flow passage sized to flow a stream with sufficient velocity to penetrate to the bottom of the powder body. The cartridge need not be evacuated prior to flowing monomer liquid into the powder.
Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings illustrating the invention, of which there are three sheets and two embodiments.